This invention relates to an injection molding machine which may selectively accommodate and use a first injection mold assembly having a central injection opening and a central mold cavity as well as at least one additional, second injection molding assembly which has an off-center injection opening for an asymmetrical injection.
The injection molding machine has a machine bed on which there is supported a mold closing unit as well as an injection molding unit. The latter may assume a first working position from which it may be moved into engagement with the first injection mold assembly lying in the central injection axis, aligned with the symmetry axis of the mold-closing unit. The injection molding unit may be moved against the first injection mold assembly by means of a hydraulic drive cylinder through a passage provided in the stationary mold carrier.
The injection molding unit may be displaced from the first working position into further working positions in which it may be moved selectively into engagement with additional injection mold assemblies with off-center injection axes and off-center injection openings, for an injection along an off-center injection axis.
The injection molding machine has a transposing mechanism to displace the injection molding unit parallel to itself from the first working position into the additional working positions in a transition plane containing the central injection axis.
In a known injection molding machines of the above-outlined type, as disclosed in German Gebrauchsmuster No. (utility patent) 1,860,806, a central injection for a first mold assembly or a non-central injection for a second mold assembly (having a non-central injection opening and a central mold cavity) is arbitrarily selectively feasible. In this manner the less utilized non-central injection process which is preferably utilized for making large-surface synthetic parts by an angled lateral injection may be used in the same injection molding machine without the need for pivoting the injection molding unit. The injection molding unit of the prior art is, together with the platform which supports it, moved into the desired injection position for the non-central injection by shifting in a horizontal and/or vertical direction. The hydraulic drive cylinders which effect the parallel shift are situated underneath the platform and are affixed directly thereto. This construction is disadvantageous in that it requires a relatively tall shifting arrangement. Consequently, the force with which the injection nozzle is urged against the injection mold assembly is introduced into the machine bed at such a height level that in case of a non-central injection opening, such level is at a relatively large distance from the axis of injection. This results in significant tilting torques during the insertion of the injection molding unit onto and its removal from the injection mold assembly. Such tilting torques have a significantly adverse effect on the accuracy of alignment of the nozzle with the injection opening of the mold assembly. In inaccurate alignment manifests itself in fluctuations in the flow passage cross section between the nozzle opening and the injection opening and thus irregularities in the injection speed occur. This is particularly the case if the nozzle assembly has no selfcentering means. Furthermore, the known construction does not permit a truly high-precision injection which requires an exactly reproducible injection axis for each cycle of injection and also for any change in the conditions of injection or changes of materials to be utilized. It is a further disadvantage of the construction according to the prior art that it makes the required maintenance work on the plasticizing cylinders after longer service periods cumbersome. This applies particularly to the removal of the feed screw, and it also applies to the emptying injection stroke of the injection molding unit when the material utilized is to be changed. This involves relatively complex procedures and is time-consuming inasmuch as with the change of the type of injection (central or non-central injection) often a change in material is involved which, in turn, requires a cleaning or replacement of the plasticizing cylinder.
In other known injection molding machines of the aboveoutlined type, as disclosed, for example, in Austrian Patent No. 315,470, as well as German Patent Nos. 3,612,587 and 3,612,588 one may selectively use for a first working position of the injection molding unit a first injection mold assembly having an injection opening lying in the central injection axis and for an additional working position an additional injection mold assembly having an injection opening through which material is injected into the parting plane of the mold assembly. The injection axis of the additional working position is oriented perpendicularly to the central injection axis of the first working position. The injection molding unit is pivotal by means of a pivoting mechanism from the first working position into the second working position. The operation in the second working position is conventionally referred to as "linear injection". Such linear injection makes possible an injection which is lateral (thus, asymmetrical) to the mold cavity while, at the same time, the injection mold cavity may be arranged symmetrically with the central axis of the second injection mold assembly. In this connection, reference is also made to FIG. 19 of a monograph by Karl Morwald, entitled "Einblick in die Konstruktion von Spritzgusswerkzeugen"("Brief Survey of the Construction of Injection Molding Tools") issued December 1965. Such a result, to be sure, may also be achieved in a first working position which is transverse to the parting plane of the injection mold assembly in case the injection mold cavity comprises a plurality of individual cavities which are arranged symmetrically about a central injection opening in the mold. Reference is made in this connection to FIGS. 16 and 17 of the monograph. The possibility of such an arrangement, however, is no longer present in case the article to be made by the injection molding requires, because of its relative dimensions, an injection mold cavity which occupies the entire width or height of the injection mold assembly. In such a case, some articles of specific configuration may only be made by a "linear injection" to ensure an acceptable quality. This applies, for example, to optical lenses in which a central (thus, symmetrical) injection by virtue of a break-off of the burr may cause damages to the lens surface in the critical zone of the axis of symmetry. A "linear injection" is further expedient in case of extremely slender, narrow articles made of highly transparent synthetic material used, for example, as observation windows in plastic containers. For such articles a "linear injection" leads to optically more favorable results. Furthermore, in case of significantly elongate, narrow articles (for example, rulers) made of fiber-reinforced synthetic material, a "linear injection" makes possible a uniform orientation of the fibers over the entire surface which would not be the case if a central, symmetrical injection is performed, because in the injection zone there would occur a disorientation of the fibers and thus a reduction of the breakage resistance in that zone.
It is of importance in the above-discussed arrangements that the mold cavity and thus the mold separating forces are capable of being oriented approximately symmetrically to the central axis of the injection mold assembly during injection.
Furthermore, it has been known for decades to arrange the injection molding unit selectively to effect a central, symmetric injection transversely to the parting plane of the injection mold assembly or a "linear injection" in the parting plane. In this connection reference is made to U.S. Pat. Nos. 3,086,244 and 3,836,301 as well as German Auslegeschrift (examined published application) No. 1,256,364. Because of the fact that the selective working positions are effected by changing the position of the injection molding unit by means of a hoisting mechanism, such an operation is not adapted to be automated. Even if the different working positions are attainable by means of a drivable pivotal mechanism, such arrangement is complex and involves considerable expense. Further, for example, a vertical orientation of the injection molding unit for the "linear injection" into the parting plane of the injection mold assembly requires, in case of large injection molding machines, a very substantially vertical space which often is not available. Even a horizontal injection molding unit which injects into the parting plane requires a very significant space for the injection molding machine because the injection axis extends significantly beyond the support surface of the machine transversely to the length thereof.